1. Field of the Invention
The invention relates to a rotational speed control for electric motors of electric hand tools having a push-button or actuating dial, the respectively linear or arcuate displacement of which acts on an actuating element in order to vary an electrical output variable, with an electronic switch for speed control of the electric motor being connected downstream of the actuating element.
2. Background of The Art
Electric tools, such as drills, drill hammers, saber saws, orbital sanders, etc. have been equipped for several years with electronic switches which permit a continuous variation of the speed. In these tools, an actuating element which brings about a change of an electrical output variable is constantly actuated in use via a push-button or via an actuating dial. The actuating element used has heretofore always been a potentiometer which is constructed as part of the electronic switch or as part of an electronic module. The push-button or the actuating dial of the electronic switch is then provided with a potentiometer slide which, as a result of the linear displacement of the push-button or the circular displacement of the dial, slides on a correspondingly shaped potentiometer path. Movement of the potentiometer slide along a linear or non-linear (e.g., logarithmic) potentiometer path in the case of the push-button embodiment or a circular potentiometer path in the case of the actuating dial embodiment brings about a corresponding change in resistance. The respective momentary resistance value serves as a manipulated electrical variable transmitted to the subsequent control electronics which brings about a speed control of the electric motor, for example, via a phase control. In this respect, the potentiometer supplies a voltage, current or resistance which is proportional to the displacement of the push-button or actuating dial.
Such potentiometers are thus subjected to a large degree of wear in use due to abrasion, dust, vibration, etc. which reduces the useful life of the electric hand tool. Furthermore, only potentiometers with resistance values which have relatively high tolerances can be economically produced in large numbers using presently used production methods. In this process, the carrier material used, e.g., hard paper, ceramics, and the resistance paste used essentially determine the quality and the price of the potentiometer.
Thus, it is an object of the present invention to provide a rotational speed control for electric hand tools which avoids the previously mentioned disadvantages by dispensing with the use of a potentiometer and employing instead an actuating element including a sensor element which is largely made of non-wearing parts and which is part of a sensor circuit.
It is a further object of the present invention to effectively use the rotational speed control above to control the rotational speed of an electric hand tool motor.